1. Field of the Invention
The present invention relates to a rodless power cylinder having inner and outer seal bands for sealing a slit on the cylinder barrel.
2. Description of the Related Art
A rodless power cylinder includes a cylinder barrel having an axial slit on the wall and a piston disposed in the bore of the cylinder barrel. The bore of the cylinder barrel is divided by the piston and forms two pressure chambers on both side of the piston. The piston is moved within the bore by introducing pressurized fluid into the pressure chambers. The movement of the piston is transferred to an external carriage by a coupling member which connects the carriage to the piston through the slit of the cylinder barrel. Usually, an inner seal band and an outer seal band covering the slit from inside and outside of the cylinder barrel are provided in order to prevent leakage of the pressure fluid the cylinder bore and an incursion of dust into the cylinder bore.
The rodless power cylinder of this type is, for example, disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-65906.
The rodless power cylinder in the '906 publication has an inner seal band covering the opening of the slit on the wall of the bore of the cylinder barrel and a dust seal band (an outer seal band) covering the opening of the slit on the outer surface of the cylinder barrel. The inner seal band has lip portions for preventing the leakage of the fluid by contacting the inner surface of the bore. Further, the inner seal band in the '906 publication is provided with an additional land near the end of the each lip portion. By providing the additional lands on the inner seal band, the fluid leaking from the lip portions due to insufficient seal function thereof is trapped in the space between the lands and the ends of the lip portions. Namely, the inner seal band of the '906 publication has a dual-seal construction to improve a seal performance. The both ends of the inner seal band are secured to end caps (end plates) disposed on both ends of the cylinder barrel, and the outsides of the end caps are covered by additional end caps. A small amount of the fluid leaking from the lip portions of the inner seal band and trapped in the space between the ends of the lip portions and the lands can escape to the outside of the cylinder barrel through small clearances between the inner seal band and the end caps or the end caps and the additional end caps.
Japanese Unexamined Patent Publication (Kokai) No. 5-106612 discloses a rodless power cylinder of similar type. The inner and outer packing belts (seal bands) are both secured by blocks fixed to the both ends of the cylinder tube (cylinder barrel). The space between the packing belts communicates with the outside of the cylinder barrel via a hole disposed on the block. The pressure in the space is maintained at a negative pressure or a positive pressure by connecting the space to an appropriate positive or negative pressure source through the hole of the blocks. When the pressure in the space between the inner and outer packing belts is maintained at a positive pressure by supplying pressurized air through the hole, the incursion of dust into the space is prevented. When the pressure in the space between the inner and outer packing belt is maintained at a negative pressure by extracting air in the space through the hole, contaminants contained in the pressurized air supplied to the bore of the cylinder do not leak to the atmosphere.
Further, Japanese Unexamined Patent Publication (Kokai) No. 3-4005 discloses a rodless power cylinder in which the end portions of the inner seal band and the outer seal band are fixed to the end caps of the cylinder barrel by inserting the end portions into holes penetrating the end caps of the cylinder barrel. As a result, the space between the seal bands is directly open to the atmosphere through the holes in the end caps.
However, the '906 publication considers the case where a very small amount of fluid leaks from the cylinder bore through the inner seal band. Therefore, if a large amount of the fluid flows into the space between the land and the end of the lip portion, for example, due to a failure of the lip seal portion, the fluid flows into the space between the inner seal band and the outer seal band. This causes a pressure rise in the space between the inner and outer seal bands and, if the pressure in the space exceeds a certain pressure, the outer seal band is blown off by the pressure in the space. In this case, the rodless power cylinder is damaged.
In the rodless power cylinder in the '612 publication, the time required for the pressure to increase in the space between the inner and the outer packing belt may become longer if the space is maintained at a negative pressure. However, if a large amount of air flows into the space between the packing belts, the pressure in the space increases and a blow off of the outer packing belt occurs.
In the rodless power cylinder of the '005 publication, since the space between the seal bands directly opens to the atmosphere, a blow off of the outer seal belt does not occur even if the inner seal band fails. In this case, however, an incursion of dust into the space between the seal band through the holes in the end caps occurs and seal bands may be damaged by the dust.